Any fossil fuel power (energy) generation system involves combustion of fuel/air mixture. Depending on means of the fuel energy conversion into electricity at least two major types of power plants exist, that exploit technologies known as steam and gas cycles.
In the steam cycle energy released in combustion process is used to generate steam, which thermal energy undergoes two-stage conversion into mechanical and electrical energy. In the gas cycle the energy of the pressurized combustion products converts directly into mechanical energy and then into electricity. The subject of this invention deals directly with the steam cycle and the dual steam-and-gas cycles (a.k.a. combined cycle) where steam cycle is a part.
Steam power plants currently dominate power generation world wide. In this cycle chemical energy of fuel is released by means of combustion within the pressurized vessel (boiler) cooled by high pressure water (working media). High pressure water converts into steam, which expands to turn steam turbine and drive a generator for production of electricity.
Upon exit from the turbine steam undergoes condensing to allow an economic utilization of the purified water. During condensing a major portion of steam energy in the form of latent heat is lost. This energy loss in the steam cycle amounts to 40%-45% of the fuel energy. Second major source of energy losses in the cycle is the energy of the combustion products (or flue gas) leaving boiler. Energy contained in the combustion products depends on type of fuel fired (gas, coal, oil) and in average for modern boilers ranges from 12% to 18% of fuel energy. There are other losses due to friction, incomplete combustion, losses to surroundings, etc. that amounts to 5% to 7%. As the result the efficiency of fuel-to-power energy conversion for the modern power plant peaks at about 35% and for average US steam power plant is about 33%.
These relatively high efficiencies were achieved by the prior art by means of fuel energy recuperation, regenerative water heating, and steam reheat. The fuel energy recuperation is achieved through water and combustion air preheat before they supplied to the boiler furnace where water boiling actually takes place. This preheat is provided within an integral part of the boiler called Heat Recovery Area. Further heat recovery from combustion products is restricted by multiple reasons. The major one is a moisture condensation from combustion products when cooled below dew point. Moisture in combustion products resulted from combustion of hydrogen present in a hydrocarbon fuel, and moisture content in the fuel and air. When such condensation occurs in the presence of carbon and sulfur dioxide it produces high concentration acid that attacks heat transfer surface or exhaust stack material. Therefore, costly materials are required to prevent substantial equipment damage and loss of operational time (plant availability and operation reliability).
The invention relates to the fossil fuel power generating systems and specifically to a high performance system having a primarily embodiment of the power plant realizing a simple steam cycle.
The conventional, simple steam cycle dominates power industry. However, the efficiency of the fuel-to-electric power conversion in the simple steam cycle on average is around 35% and for most advanced systems does not exceed 40% relative to the higher heating value of the fuel. The major reason for the low fuel energy-to-electricity conversion is the intrinsic heat loss with a latent heat of the working media in the condenser (about 50%). The second most significant reason is the heat loss with the combustion products in the stack, which may vary from 10% to 20% depending on conditions of operation and fuel fired. Yet, an another drawback associated with operation of fossil fuel power plant is the atmosphere pollution with carcinogenic nitrogen oxides. The low efficiency of the simple steam cycle is also responsible for an excessive carbon dioxide emission—the major part of green house gases. Consider the limitations of the prior art in the details.
Since commercial introduction of the simple steam cycle, in the middle of 19th century, its efficiency improvement had two major pathways, such as 1)—regenerative feedwater heating by heat recovery from the working media passing through steam turbine, and 2)—heat recovery from the combustion products. Another valuable avenue for fuel-to-energy efficiency improvement includes steam parameters increase and steam reheat. Although these means do not affect the essence of this invention and therefore they will not be taken into consideration hereafter.
Before introduction of the regenerative feedwater heating (hereafter also referred to as regenerative duties), a significant loss of the thermal energy with the latent heat of the working media occurred in condenser. After adoption of the regenerative duties (a.k.a. regenerative steam cycle), the steam extraction from the various stages of turbine provided heat credits to the condensate of working media as it is pumped back to the boiler. Almost 20% of cycle heat loss reduction due to use of the regenerative duties can be estimated by a comparison of the area representing unavailable heat for ideal steam cycles. The regenerative duties have certain optimum values as defined by the number of regenerative heaters is determined by the parameters of the working media and typically, therefore establishing some specific limits which and is selected to maximize the steam cycle efficiency has maximum value. Deviation from such optimized conditions results in the decline of fuel-to-power conversion.
Another most significant source of heat loss that affects fuel consumption, i.e. the efficiency of fuel-to-energy conversion, is the heat loss of the sensible and latent heat with the combustion products rejected to the atmosphere. These losses, accounting for up to 10%-18% of fuel energy loss, have long been tolerated for a variety of engineering and economic reasons.
On the one hand, a low convection heat transfer coefficient and the low temperature of the flue gas would require large heat exchangers. At the same time, the high moisture in the combustion products would require expensive corrosion-resistant material. Additionally, the presence of sulfur in the fuel will further aggravate the heat exchangers operating conditions, resulting in reduced boiler reliability and increased maintenance costs. Therefore, thanks to the abundance of fossil fuel, and its low price in the past, there were no strong economic incentives to address these losses. It is this combination, which has prevented engineering and operating companies from further pursuing fuel utilization improvement by waste heat recovery from combustion products rejected to the atmosphere.
Losses with the combustion products rejected into atmosphere (a.k.a. stack losses) for the typical operating conditions of the simple steam cycle for power/industrial applications. The losses are distinguished by the source, specifically the fraction of losses due to moisture content (including latent heat), and the sensible heat content of the dry flue gas.
Economics of large utility and industrial-size boilers has stimulated the most efficient utilization of fuel in the boilers, especially for the electric power generation. Here the heat leaving boiler furnace is recovered by means of the feedwater (economizer) and combustion air preheating. However, further heat recovery is prevented mostly by the rate of heat transfer surface corrosion due to moisture condensation from the flue gas. Typical exit flue gas temperature for power/industrial steam generators is approximately 330° F. to 350° F. to provide a necessary margin above flue gas dew point. Even with this margin, corrosion is frequently a problem at the gas side of the air-heater exit, due to low temperature of the combustion air supply. This need for margin eventually determines the level of the heat losses with the exit flue gas.
To minimize this margin steam is often used to preheat combustion air before entering the forced draft fan in order to avoid air-heater damage. In turn, such use of steam also leads to the power generation loss or further reduction of the fuel-to-energy conversion efficiency, which is accepted due to offset the cost of maintenance and repair.
During the past 30 years, the United States and other industrialized and developed countries have been working to improve fuel efficiency, related to boiler exit flue gas losses. Some solutions were proposed that involved the heat transfer surface made of corrosion resistant materials such as quartz, or thermally resistant glass, or stainless steel. However, high capital and operating costs, as well as reliability and maintenance issues, ultimately led to rejection of this solution by operating companies.
The lack of interest toward energy recovery from the combustion products rejected to the atmosphere especially for the stand alone applications is also explained by its low-grade energy. If implemented it could be used only as the replacement of the conventional duties, therefore representing an unnecessary trade-off with the marginal or even questionable cycle duty improvement. Thus, recovery of the rejected heat with the existing concepts of the simple steam cycle for the stand-alone plants does not present a viable option from both engineering and economical stand point, therefore making these losses unavoidable.
Environmental aspects of the simple plant performance also present many challenges concerning NOx emissions reduction. The in-furnace emissions suppression is typically achieved by means of ultra-low NOx burners along with deep air/oxidizer staging provided by over-fire air ports. However, these means alone do not suffice the environmental regulations. Therefore, to meet stringent environmental regulations for NOx emissions suppression, the natural gas and coal re-burn where introduced where portion of fuel was injected in furnace between the main combustion and burnout zone. The latter one is often called the over fire air zone. The re-burn technologies are also competing with very efficient post-combustion technologies such as selective catalytic (SCR), non-catalytic (SNCR), or the combinations of above that bring NOx emissions to very low level to meet or even surpass current environmental regulations. However, both re-burn and the post-combustion systems incur significant capital and operating expenses, therefore adversely influencing the economics of fossil power plant operation.
In addition, low efficiency of fuel-to-electric power conversion is also responsible for essential carbon dioxide emissions therefore forcing seriously consider an expensive CO2 sequestration options. For this reason, significant efficiency improvement is the economically preferred way of coping with carbon dioxide emissions and hazardous NOx rate reduction.